The SEP wouldn't propel the crew vehicles but would move various support systems: A SEP system weighs a lot less than chemical propulsion. Given enough time it can move a lot of mass through space. That in-space advantage makes it particularly attractive for pre-positioning cargo—supplies, habitats and the like—to keep human explorers alive after they arrive on a faster vehicle to explore a distant location. Some additional info:/-- NASA Selects Companies To Study Solar Electric Propulsion Spacecraft - NASA - Nov.13.11/-- NASA Seeks Game Changing Solar Array Systems Proposals - NASA - April.26.12/-- Solar Electric Propulsion Could Slash the Price of Manned Missions - AmericaSpace - Aug.1.12

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